This invention relates to a delay circuitry and more particularly to a delay circuitry employing a delay line, such as a supersonic delay line, a delay time of which is unstable due to the conditions around the delay line.
A delay circuitry employing a supersonic delay line is used, for example, in a vertical aperture correction apparatus. In such a circuitry, a carrier signal generated in a carrier signal generator is modulated by an amplitude modulator with a video input signal and the modulated carrier signal is delayed through the supersonic delay line by one horizontal scanning period. The delayed video signal is obtained by detecting the modulated and delayed carrier signal with the carrier signal from the carrier generator in a synchronous detector.
Since the delay time of the supersonic delay line is not stable, but changes especially in accordance with the temperature therearound, the supersonic delay line should be soaked in a constant temperature oven to prevent any change of the delay time, if it is required to stabilize the gain of the delay circuitry. This method is effective if the carrier signal frequency is not very high. But with an increase of the carrier signal frequency, the gain becomes less stable, even if the temperature change is limited by the constant temperature oven. In a system in which the video signal has a bandwidth of about 30 MHz and a carrier frequency of about 100 MHz is required, such a delay circuitry utilizing the constant temperature oven according to the prior art cannot be used as the 1H-delay line for the vertical aperture correction apparatus because of the reduced stability of the gain of the delayed video signal. Moreover, such a delay circuitry requires the space for the constant temperature oven.